Extrafusal fibers are defined as what?

• A. Muscle fibers that create big contractions/movements
• B. Muscle fibers that sense stretch
• C. Sensory Neurons
• D. Connective Tissue Around Fiber

Answer: (A)

Extrafusal fibers are the primary contractile muscle cells that generate force for movement. When activated by alpha motor neurons, they produce the powerful contractions you rely on for activities like lifting, running, and jumping. This makes them the fibers responsible for creating large, purposeful movements. In contrast, the fibers that sense stretch are intrafusal fibers located inside the muscle spindle; they help monitor length and tension and are involved in reflex regulation rather than producing large contractions. Sensory neurons are a different cell type altogether, responsible for transmitting sensory information, not for contracting muscle. The connective tissue around a fiber (the endomysium) surrounds the muscle fiber but does not itself contract. So extrafusal fibers are defined by their role as the main force-generating, movement-causing muscle cells.